a) Field of the Invention
In stereo microscopy, in contrast to standard microscopy, two separate light channels are guided through the same objective in order to achieve a stereoscopic effect. For this reason, the dimensions of objectives of this kind are greater than those of conventional objectives with respect to the diameter of the lenses. Therefore, correction of the longitudinal chromatic aberration is particularly important because this is perceived in the stereo microscope as a transverse chromatic aberration which severely interferes with the visual impression. Other desirable characteristics of such objectives are an image field which is as flat as possible in order to prevent the so-called globe effect which allows flat surfaces appear curved, a large working distance allowing convenient handling with the sample, and a high resolution.
b) Description of the Related Art
Various objectives which more or less satisfy these conditions are known from the prior art. U.S. Pat. No. 4,640,586 describes an objective type which, in its most compact form, makes do with four lenses. The objectives are well corrected chromatically and have, in part, a large working distance of more than 100 mm. However, a clear globe effect is disadvantageously noticeable and the relative aperture or focal ratio is also relatively small. U.S. Pat. No. 6,339,507 describes an objective which has only an insufficient chromatic correction.
On the other hand, objectives are also known in the prior art which combine a high resolution and a flattened image field. However, these objectives have the disadvantage that they have only a short working distance and are very bulky. For example, JP 2001-221955 proposes a solution with excellent chromatic correction, low distortion and a large focal ratio and in which the globe effect is only slightly apparent. However, the working distance is only 32 mm maximum and the objective is constructed in a very complicated manner with eight lenses and is costly.